The decision to cut Harrington, who opened the 2007 season as the starter, leaves former Georgia player D.J. Shockley as the team's No. 3 quarterback behind rookie starter Matt Ryan and Chris Redman.
Rookie running back Thomas Brown, also of Georgia, was placed on injured reserve.
Shockley played behind Ryan and threw a 20-yard touchdown pass to Brian Finneran for the only touchdown in the Falcons' 10-9 preseason win at Baltimore on Thursday night.
Finneran, who missed the past two seasons with two reconstructive knee surgeries, survived Saturday's cuts.
The Falcons also waived veteran tight end Marcus Pollard, who signed with the team on Aug. 24.
"We are pleased about our roster heading into the 2008 season," said general manager Thomas Dimitroff.
"I'm satisfied with the process we went through to get to this point, including ... minicamp, training camp and the preseason," Dimitroff said.
"Our coaching staff did a good job guiding our players over the past five months and I'm very proud of the team our staff put together. I'm looking forward to starting the regular season."
Forney made 89 starts in his seven seasons with the Falcons. He did not miss a start in three consecutive seasons from 2004-06 and made 14 starts last season.
The Falcons also waived offensive linemen D'Anthony Batiste, Michael Butterworth, Doug Datish and Terrance Pennington and defensive linemen Willie Evans, Brandon Miller, Rashad Moore and Montavious Stanley.
Other players cut were: safeties Eric Brock, Daren Stone and Deke Cooper, cornerbacks Wilrey Fontenot and Glenn Sharpe, linebackers Mickey Pimentel and Cameron Vaughn, receivers Eric Weems and Chandler Williams and tight end Keith Zinger.